In the field of electrophotography, the copying speed required has tended to increase to meet the ever increasing quantity of information. In a high-speed copying machine, the heat transmitted from a fixing hot roll is low compared with a medium-speed copying machine; as well, the surface temperature of the fixing hot roll falls appreciably because more heat absorbed by the paper than can be fully compensated for. Accordingly, there is a demand for a toner composition which can be fixed at low heat and does not cause an offset problem at this lower level of fixing temperature.
On the other hand, copying machines have become smaller, and the pressure and temperature of a fixing roll have lowered. In this field, the development and improvement of developers with good fixing properties and offset resistance even at low heat have also been conducted.
For example, there is known a process disclosed in Japanese Patent Publication No. 6895/1980 which provides a toner having good offset resistance using, as a resin for the developer, a resin having a weight-average molecular weight/number-average molecular weight ratio of 3.5-40 and a number-average molecular weight of 2,000-30,000; and also a process disclosed in Japanese Patent Application Laid-Open No. 101031/1974 which widens the range of fixing temperature and provides a toner free from offset problems even at a comparatively high fixing temperature by using a crosslinked resin.
It has been found, however, that whereas the prior art as described above is effective for the conventional copying machine troubled with an offset problem, a sufficient effect cannot be obtained in the case of a copying machine which uses a lower heat for fixing. This is presumed to result from the improvements that have been made in order to increase the weight-average molecular weight of the resin to achieve an improvement in the strength and offset resistance of the toner.
Accordingly, the resins described above have such a high viscosity that they are not suitable in their conventional form for the lower-heat fixing copying machines. Viscosity reduction of the resin by lowering its weight-average molecular weight, however, tends to damage the offset resistance or the strength of the toner. As the result of a long-term operation, breakage of the toner occurs, which tends to deteriorate the picture quality.